9 Garbage-First Garbage Collector

G1 partitions the heap into a set of equally sized heap regions, each a contiguous range of virtual memory as shown in Figure 9-1. A region is the unit of memory allocation and memory reclamation. At any given time, each of these regions can be empty (light gray), or assigned to a particular generation, young or old. As requests for memory comes in, the memory manager hands out free regions. The memory manager assigns them to a generation and then returns them to the application as free space into which it can allocate itself.

Figure 9-1 G1 Garbage Collector Heap Layout

Description of "Figure 9-1 G1 Garbage Collector Heap Layout "

The young generation contains eden regions (red) and survivor regions (red with "S"). These regions provide the same function as the respective contiguous spaces in other collectors, with the difference that in G1 these regions are typically laid out in a noncontiguous pattern in memory. Old regions (light blue) make up the old generation. Old generation regions may be humongous (light blue with "H") for objects that span multiple regions.

An application always allocates into a young generation, that is, eden regions, with the exception of humongous, objects that are directly allocated as belonging to the old generation.

G1 garbage collection pauses can reclaim space in the young generation as a whole, and any additional set of old generation regions at any collection pause. During the pause G1 copies objects from this collection set to one or more different regions in the heap. The destination region for an object depends on the source region of that object: the entire young generation is copied into either survivor or old regions, and objects from old regions to other, different old regions using aging.

On a high level, the G1 collector alternates between two phases. The young-only phase contains garbage collections that fill up the currently available memory with objects in the old generation gradually. The space-reclamation phase is where G1 reclaims space in the old generation incrementally, in addition to handling the young generation. Then the cycle restarts with a young-only phase.

Figure 9-2 gives an overview about this cycle with an example of the sequence of garbage collection pauses that could occur:

Figure 9-2 Garbage Collection Cycle Overview

Description of "Figure 9-2 Garbage Collection Cycle Overview "

The following list describes the phases, their pauses and the transition between the phases of the G1 garbage collection cycle in detail:

1. Young-only phase: This phase starts with a few young-only collections that promote objects into the old generation. The transition between the young-only phase and the space-reclamation phase starts when the old generation occupancy reaches a certain threshold, the Initiating Heap Occupancy threshold. At this time, G1 schedules an Initial Mark young-only collection instead of a regular young-only collection.

   • Initial Mark : This type of collection starts the marking process in addition to performing a regular young-only collection. Concurrent marking determines all currently reachable (live) objects in the old generation regions to be kept for the following space-reclamation phase. While marking hasn’t completely finished, regular young collections may occur. Marking finishes with two special stop-the-world pauses: Remark and Cleanup.

   • Remark: This pause finalizes the marking itself, and performs global reference processing and class unloading. Between Remark and Cleanup G1 calculates a summary of the liveness information concurrently, which will be finalized and used in the Cleanup pause to update internal data structures.

   • Cleanup: This pause also reclaims completely empty regions, and determines whether a space-reclamation phase will actually follow. If a space-reclamation phase follows, the young-only phase completes with a single young-only collection.

2. Space-reclamation phase: This phase consists of multiple mixed collections that in addition to young generation regions, also evacuate live objects of sets of old generation regions. The space-reclamation phase ends when G1 determines that evacuating more old generation regions wouldn't yield enough free space worth the effort.

After space-reclamation, the collection cycle restarts with another young-only phase. As backup, if the application runs out of memory while gathering liveness information, G1 performs an in-place stop-the-world full heap compaction (Full GC) like other collectors.

The Initiating Heap Occupancy Percent (IHOP) is the threshold at which an Initial Mark collection is triggered and it is defined as a percentage of the old generation size.

G1 by default automatically determines an optimal IHOP by observing how long marking takes and how much memory is typically allocated in the old generation during marking cycles. This feature is called Adaptive IHOP. If this feature is active, then the option -XX:InitiatingHeapOccupancyPercent determines the initial value as a percentage of the size of the current old generation as long as there aren't enough observations to make a good prediction of the Initiating Heap Occupancy threshold. Turn off this behavior of G1 using the option-XX:-G1UseAdaptiveIHOP. In this case, the value of -XX:InitiatingHeapOccupancyPercent always determines this threshold.

Internally, Adaptive IHOP tries to set the Initiating Heap Occupancy so that the first mixed garbage collection of the space-reclamation phase starts when the old generation occupancy is at a current maximum old generation size minus the value of -XX:G1HeapReservePercent as the extra buffer.

G1 marking uses an algorithm called Snapshot-At-The-Beginning (SATB) . It takes a virtual snapshot of the heap at the time of the Initial Mark pause, when all objects that were live at the start of marking are considered live for the remainder of marking. This means that objects that become dead (unreachable) during marking are still considered live for the purpose of space-reclamation (with some exceptions). This may cause some additional memory wrongly retained compared to other collectors. However, SATB potentially provides better latency during the Remark pause. The too conservatively considered live objects during that marking will be reclaimed during the next marking. See the Garbage-First Garbage Collector Tuning topic for more information about problems with marking.

When the application keeps alive so much memory so that an evacuation can't find enough space to copy to, an evacuation failure occurs. Evacuation failure means that G1 tries to complete the current garbage collection by keeping any objects that have already been moved in their new location, and not copying any not yet moved objects, only adjusting references between the object. Evacuation failure may incur some additional overhead, but generally should be as fast as other young collections. After this garbage collection with the evacuation failure, G1 will resume the application as normal without any other measures. G1 assumes that the evacuation failure occurred close to the end of the garbage collection; that is, most objects were already moved and there is enough space left to continue running the application until marking completes and space-reclamation starts.

If this assumption doesn’t hold, then G1 will eventually schedule a Full GC. This type of collection performs in-place compaction of the entire heap. This might be very slow.

See Garbage-First Garbage Collector Tuning for more information about problems with allocation failure or Full GC's before signalling out of memory.

Humongous objects are objects larger or equal the size of half a region. The current region size is determined ergonomically as described in the Ergonomic Defaults for G1 GC section, unless set using the -XX:G1HeapRegionSize option.

During the young-only phase, the set of regions to collect (collection set), consists only of young generation regions. G1 always sizes the young generation at the end of a young-only collection. This way, G1 can meet the pause time goals that were set using -XX:MaxGCPauseTimeMillis and -XX:PauseTimeIntervalMillis based on long-term observations of actual pause time. It takes into account how long it took young generations of similar size to evacuate. This includes information like how many objects had to be copied during collection, and how interconnected these objects had been.

If not otherwise constrained, then G1 adaptively sizes the young generation size between the values that -XX:G1NewSizePercent and -XX:G1MaxNewSizePercent determine to meet pause-time. See Garbage-First Garbage Collector Tuning  for more information about how to fix long pauses.

During the space-reclamation phase, G1 tries to maximize the amount of space that's reclaimed in the old generation in a single garbage collection pause. The size of the young generation is set to minimum allowed, typically as determined by -XX:G1NewSizePercent, and any old generation regions to reclaim space are added until G1 determines that adding further regions will exceed the pause time goal. In a particular garbage collection pause, G1 adds old generation regions in order of their reclamation efficiency, highest first, and the remaining available time to get the final collection set.

The number of old generation regions to take per garbage collection is bounded at the lower end by the number of potential candidate old generation regions (collection set candidate regions) to collect, divided by the length of the space-reclamation phase as determined by -XX:G1MixedGCCountTarget. The collection set candidate regions are all old generation regions that have an occupancy that's lower than -XX:G1MixedGCLiveThresholdPercent at the start of the phase.

The phase ends when the remaining amount of space that can be reclaimed in the collection set candidate regions is less than the percentage set by -XX:G1HeapWastePercent.

See Garbage-First Garbage Collector Tuning for more information about how many old generation regions G1 will use and how to avoid long mixed collection pauses.